An electrophoretic display device displays character data, image data, and the like, by the behavior of electrophoretic particles, for example, when a voltage is applied to a dispersion in which the electrophoretic particles are dispersed in a solvent. For example, if the electrophoretic particles and the solvent are colored with different colors, the color of the electrophoretic particles is observed when the electrophoretic particles are moved to the surface of the solvent by voltage application, and the color of the dispersion is observed when the electrophoretic particles are moved to the bottom of the solvent. If electrodes to which voltage application is made possible by address appointment are provided, different colors for the respective addresses can be displayed, and accordingly, arbitrary character data and image data can be displayed. Moreover, it is possible to rewrite the display data, and at the same time, it is advantageous that the display data can be maintained as it is, even if there is no electric signal.
In recent years, in place of the conventional electrophoretic display devices (e.g., see Japanese Patent Publication No. 50-15115) in which a dispersion of electrophoretic particles are enclosed in spaces between opposed electrode substrates, there have been developed microcapsule type electrophoretic display devices (e.g., see Japanese Patent No. 2551783) with a structure in which microcapsules enclosing a dispersion of electrophoretic particles are arranged between opposed electrode substrates. As compared with the conventional electrophoretic display devices, the microcapsule type electrophoretic display devices are remarkably improved in various performances and functions, such as long-term stability of display, responsiveness, contrast, and display rewritable times.
In general, as a method of preparing microcapsules each enclosing a core substance, since it is advantageous that the strength and thickness of shells can easily be controlled and a shell with two or more layers can be formed, a coacervation method (e.g., see U.S. Pat. No. 2,800,457) is generally said to be preferred. Also in the field of electrophoretic display devices, for example, the research and development of microcapsules each enclosing a dispersion of electrophoretic particles in a shell composed of gelatin and gum arabic as essential components have been carried out using the coacervation method.
However, with respect to an electrophoretic display device using such microcapsules, there is a problem that when the electrophoretic display device is left for a long period of time under high temperature and high humidity conditions (e.g., at 60° C. and 90% RH for 24 hours), the contrast of the electrophoretic display device after that is found to be remarkably decreased.
Thus, the Japanese Patent Laid-open Publication No. 2005-338189 discloses microcapsules for electrophoretic display devices, each enclosing a dispersion of electrophoretic particles in a shell obtained by the reaction of a specific water-soluble surfactant with a compound having an epoxy group or a thiosulfide group, as microcapsules for electrophoretic display devices, which can suppress a decrease in the contrast even after the electrophoretic display devices are left for a long period of time under high temperature and high humidity conditions (e.g., at 60° C. and 90% RH for 24 hours).